Extrusion of thermoplastic films and articles made therefrom



Dec. 9, I K. THOMPSON ET AL EXTRUSION 0F THERMOPLASTIC FILMS ANDARTICLES MADE THEREFROM Filed Feb. 25, 1966 5 Sheets-Sheet 1 3,482,278THEREFROM Dec. 9, 1969 K. THOMPSON ET Al.

EXTRUSION OF THERMOPLASTIC FILMS AND ARTICLES MADE Filed Feb. 25, 1966 5sheets-sheet 2 Ll hw MINNI A o n. OMQQ @Q 0.@

D 1969 ETA 4 ec 9, K. THoMPsoN L A 3,482,278

EXTRUSION OF' THERMOPLASTIC FILMS AND ARTICLES MADE THEREFROM Filed Feb.25, 1966 5 Sheets-SheetI 3 Dec. 9, 1969 K. THoMPsoN ET L 3,482,278

EXTRUSION OF THERMOPLASTIC FILMS AND ARTICLES MADE THEREFROM Filed Feb.25; 1966 5 Sheets-Sheet 4 Dec. 9, 1969 K. THOMPSON ET Ax. 3,482,278

I EXTRUSION OF THERMOPLASTIC FILMS AND'ARTICLES MADE THEREFROMFiled'Feb. 25, 1966 5 Sheets-Sheet 5 'y ll?" *.14

30 l@ ./f WMM @7m/W United States Patent O EXTRUSION F THERMOPLASTICFILMS AND ARTICLES MADE THEREFROM Kenneth Thompson, Waynesville, N.C.,and Richard C.

Ihde, Parma Heights, Ohio, assigmors to Champion Papers Inc., Hamilton,Ohio, a corporation of Ohio Filed Feb. 25, 1966, Ser. No. 530,119

Int. Cl. B291? 3/02; B29d 9/00 U.S. Cl. 18-12 7 Claims ABSTRACT OF THEDISCLOSURE A die structure of extruding a thin thermoplastic film ontopaperboard, the die structure having uniformly spaced die edges andhaving recesses extending upwardly from the die edges to affect theextrusion of a thick strip of film from the die edges adjacent therecesses.

This invention relates to the extrusion of plastic film and particularlyto the extrusion of plastic to provide a film having a profiled crosssection, that is, a cross section in which certain preselected portionsare thicker than other portions. The invention also relates to themanufacture of articles using the profile film of the present invention.

The invention will be described in particular relation to paperboardmilk cartons of the type described in Patent 3,120,335 but it will beapparent that the invention has application to other manufactures.

In general, the milk carton on which the invention is employed ispaperboard coated on lboth sides with a thermoplastic material such aspolyethylene which is extruded onto the board as the board passes in theform of a web adjacent the extruder. The coated board is normallyprinted, scored, cut into blanks, sealed along one edge to form a tube,and sealed at its bottom and top ends. Probably the most criticalportion of the milk carton is its bottom structure including all bottomscores and corners, for it is this portion of the milk carton which mostfrequently develops leaks which render the carton unsuitable for the enduse.

It is desired to minimize the leaking of paperboard cartons caused bypinholes and bottom failures of the type described above by increasingthe thickness of the thermoplastic coating on the bottom portion of thecarton. It is, of course, possible to attain the objective of minimizingleaks by markedly increasing the thickness of the film all over thecarton ibut such an increased thickness of film in areas where it is notneeded would be uneconomical.

Therefore, it has been an objective of the invention to provideapparatus for extruding a plastic film in which the film has preselectedareas of varying thickness. In a principal application of the inventionthe -film s extruded onto paperboard which is later converted to milkcartons, with the thicker film overlying the bottom portion of the milkcarton.

In extruding generally, when it is desired to extrude a particularprofile, the extruder orifice is shaped to the desired configuration inorder to produce the required extrusion section. In extruding plasticfilms, the profiling of the film is not that simple because of thesurface tension effects acting on an extremely thin fluid film. If, forexample, a thick strip of rectangular configuration is to be extruded asa part of a thinner lfilm, the desired profile cannot be obtained bysimply forming the die orifice in the shape desired for the rectangularstrip in the film wall, through surface tension effects, assume aconfiguration different from that of the die orifice, the configuraicetion having a thin line of reduced thickness much below the minimumthickness permitted.

An objective of the present invention has been to provide for theextrusion of a thick generally rectangular cross section strip in a thinfilm, the strip being of substantially uniform thickness across itswidth. This objective is attained by providing for a strip of generallyelliptical cross section issuing immediately from the exit of theextrusion die and permitting the surface tension effects to cause agathering of the molten plastic along the longitudinal edges of thestrip to form the corners of the strip without any objectionablediminution of the thickness of the strip across its width or thethickness of adjacent film areas.

It has been another objective of the invention to provide an extrusiondie having elongated substantially uniformly spaced die edges and ashort recess formed adjacent at least one of said die edges, the recesscausing an elliptically profiled strip, as a part of a film, to beformed immediately adjacent the exit side of the die.

Another objective of the invention has been to provide a. die forextruding thermoplastic films having strips differing thicknesses alongthe width of the film, the die having elongated substantially uniformlyspaced die edges and having a cavity behind the die edges which isgenerally trianugular in transverse section, the cavity extending alongthat portion of the die :which is to produce the thicker strips of film.It has been found that the triangular cavity lowers the pressure drop ofthe plastic fiuid flowing to the die edges and that this permits a goodcontrol of the ratio of the volume issuing adjacent the recessed andnonrecessed areas, respectively. yMore specifically, if the recessedportion of the die causes approximately twice as much plastic per unitof die length to issue as compared to the non-recessed portions of thedie, that 2:1 ratio will remain constant through a wide range of totalplastic volume issuing through the die.

These and other objectives of the invention will become more readilyapparent from the following detailed description taken in conjunctionwith the following drawings in which:

FIG. 1 is a perspective view of a milk carton employing the invention,

FIG. 2 is a plan view of a web to which the coating of the presentinvention has been applied,

FIG. 3 is a cross sectional view taken along lines 3-3 of FIG. 2,

FIG. 4 is a diagrammatic side elevational view illustrating the layoutwith which the extruder of the present invention is used,

FIG. 5 is a top plan view of the extruder die assembly,

FIG. 6 is a side elevational view of the die assembly,

FIG. 7 is an end elevational view of the die assembly,

FIG. 8 is a cross sectional View taken along lines 8-8 of FIG. 6,

FIG. 9 is a cross sectional view taken along lines 9-9 of FIG. 6,

FIG. l0 is a fragmentary enlarged view of the encircled portion of FIG.9,

FIG. 1l is a cross sectional view taken along lines 11--11 of FIG. 8,

FIG. l2 is a cross sectional view taken along lines 12-12 of FIG. 8,

FIG. 13 is a fragmentary perspective view of the operative face of theadjustable die lip,

FIG. 14 is an enlarged diagrammatic cross sectional view of profiledplastic exiting from the extrusion die,

FIG. l5 is an enlarged diagrammatic cross sectional view of the profiledplastic just before it adheres to the paperboard web, and

FIG. 16 is an enlarged diagrammatic cross sectional view of the profiledplastic and paperboard laminate,

FIG. 17 isa fragmentary plan view of a carton blank`V Y which has beencoated, cut and scored."

The milk carton shown at 20 in FIG. 1 is formed of paperboard and 1iscoated with polyethylene, both inside and out. The carton details arefully disclosed in Patent No. 3,120,335. As illustrated in FIG. 1 andshown in more `detail in the patent referred to above, the carton bottomstructure is formed by folding panels 21 transversely across the bottomof the carton, the panels being folded onl score lines 29. The bottomstructure is complete-d by applying heat and pressure to the panelswhich are lfolded upon one another. The polyethylene which has beenapplied to the paperboard fuses during the application of heat andpressure to form a sealed bottom structure.

After the carton bottom structure has been formed, the carton is filledwith milk or other liquid and then the top seal is formed. Inythehandling of the carton after the forming and filling until it reachesthe place of the ultimate user, the carton is subjected to stressescaused by bouncing, vibrating, and the like which causes the contents toforce the side walls to flex. The effect of this stress is mostpronounced at the location of the fold lines 29 and it is in this areathat the greatest occurrence of i leaks is found.

The objective of the present invention has been to provide an increasedthickness of polyethylene on at least one surface of the carton,preferably the inside surface, which will cover the score line 29 andincrease the resistance of that area to that fracture which will causeleaking, that is failure, of the container. It would, of course, bepossible to increase the thickness of the polyethylene film at the scoreline simply by increasing the thickness of the polyethylene filmuniformly all over the surface of the carton. However, such an increasewould markedly -increase the cost of manufacture of the carton and is,therefore, unsuitable.

The manner in which the plastic coating is increased in the area of thescore line is illustrated in part in FIG. 2 which is a plan view of afragment of the web which is coated with the plastic. The web indicatedat 25 is, in the exemplary embodiment, wide enough to space six cartonblanks 26 across its width. After being coated the web will be slitalong line 19 and rewound into three rolls. Thereafter the carton blankswill be cut, scored, formed into cartons and filled.

The remaining lines shown on the web 25 are merely to illustrate wherethe longitudinal top forming cut lines 27, the transverse cut lines 28,major horizontal score lines 29 for the bottom structure, and the bottomforming cut lines 30 occur. As shown, the pattern of each blank 26 issuch that transversely adjacent blanks nest with each other along theirbottom forming cut lines 30. This nesting spaces the major horizontalscore lines 29 to be formed in respective carton blanks approximatelyfive inches from each other. Thus, it is possible to apply an extrathickness of plastic film to the score line areas 29 by applying athicker film of plastic on a longitudinal strip 31 which isapproximately six inches wide, the area of the thicker plastic extendingapproximately l/2 inch beyond the score lines 29.

By way of example, the paperboard substrate 25 is .022 inch thick andweighs 285 pounds per 3,000 sq. ft. ream (500 sheets at 24 x 36 inches).The profile coating in the thin area indicated at 33 is .00075 inchthick and in the thicker area indicated at 31 is .0015 inch thick. It isto be understood that all of the dimensions set yforth throughout thisapplication are merely examples of one form of the invention and aresubject to considerable variation ydepend-ing on the particularapplication to which the invention is put.

The profile coating is applied in an extruder line as illustrated inFIG. 4. There, a supply roll of paperboard stock is fed as a web overidler rolls 41 into a nip 42 formed between a chrome chill roll 43 and arubber presl sure roll 44, bothof which are water cooled. Immediatelyabove the nip is an extruder 45 to be described in greater detail belowwhich continuously feeds an extruded hot plastic film 46 into the nip42. The outlet of the extruder 45 is preferably positioned in relationto the nip so the film 46 rst engages the vweb and thereafter is pulledby the web 4into the nip where it is chilled by its engagement with thechill roll. The lineal speed of the web is approximately twenty timesfaster than the feeding rate of the extruded film. Consequently, thefilm is stretched to approximately one twentieth its thickness betweenthe time it is extruded and the time it engages the web.

The coated web 39, after it leaves the nip, passes over additional webhandling idler rolls 41 to reverse the position of its surfaces so thatthe coated surface faces down and the uncoated surface is upwardlyfacing to receive the second coating. The second coating is applied bythe extruder 50 as the web passes through the nip 51 formed between achill roll 52 and a pressure roll 53.

After passing through the nip 51, the web passes over additional idlerrolls 41 and is wound on a reel 54. Between the nip' 51 and the reel 54,thesurface 0f the web may be subjected to treatment as by a coronadischarge, for example, which conditions the polyethylene surface tomake it more receptive to printing.

It is common practice thereafter to slit the web and rewind -it intoseparate rolls which are shipped to the converter. At the converter, theweb is printed, scored, and cut into separate blanks, the blanksreceiving a side seam. In this condition, the blanks are shipped to thedairy where the bottom structure is formed, the carton is filled, andthe top structure is formed.

Extruder die assembly The extruder die assembly receives molten plasticsuch as polyethylene which has a temperature of approximately 600 F. anda pressure of 300 p.s.i. The plastic is brought to the die assembly inthat condition by the extruder structure which is shown, for example, inPatent No. 2,944,286, the details of which do not form any part of thepresent invention. The extruder receives the plastic in the form ofpellets and drives the pellets by means of a screw to the die assemblythrough a metering valve. The screw feeds the plastic at a rate greaterthan the metering valve will pass. As a consequence, the plastic notonly flows toward the metering valve but reverse its flow where it issheared between the screw thread and the housing in which the screwthread is located. This pressure and shearing action causes the plasticto reach approximately the temperature at which it is to be introducedinto the die assembly. Preferably, the screw has additional heatersmounted around its housing to add additional heat as required.

The die assembly to which the molten plastic is fed is illustrated inFIGS. 5 to 12. The die assembly is supported by four bolts on an adaptermanifold 61 which includes an inlet pipe 62 through which the moltenplastic is fed into the die assembly. The die assembly includes anelongated die body 63 on which induction heaters 64 are mounted, a fixeddie lip 65 which is secured to the body by a wedging block 66, and aseries of bolts 67 and an adjustable die lip 70 which is fixed to thedie body 63 by clampingbolts 71.

Each heater `64 includes a winding 75 Wound about an iron core 76 whichis U-shaped and has its ends passing through the insulation 77 mountedacross the upper surfaces of the die body 63. Thus, the die bodycompletes the magnetic circuit with` which each coil i'z associated andthe energizing of the vwindings with alternating current causes flux topass through the die body 63, the hysteresis loss in the body causingthe body to heat to the desired temperature. As shown irl-FIG. 8, athermocouple 78 projects into the diebody 63 to detect its temperature.

The die body has an entrance channel 80 (FIGS. 9 and 12) through whichthe molten plastic passes from inlet pipe 62 into the die body. Theoutlet 0f the channel 80 is adjacent an apex 81 of a V-shapeddistribution manifold 82 in the surface of the fixed die lip 65. Thefixed die lip 65 has a surface 83 below the distribution channel ormanifold which is recessed approximately .020 inch from the surface 84above the distribution manifold. That recess spaces the surface 83 awayfrom the opposed surface of the die body and permits molten plastic tofiow between the fixed die lip and the die body.

Below the distribution manifold 82 are grooves 85 and 86 on the fixedand adjustable die lips respectively, the grooves together forming arecovery channel in which the resin collects at equal pressure allacross the die. The recovery channel is spaced a uniform distance fromthe die exit 87. Below the recovery channel, the facing surfaces 88 and89 of the die lips are planar, with the exceptions described below, andare spaced apart approximately .020 inch. With all conditions beingequal, the plastic should ow from the recovery channel out of the exitat a uniform rate all across the exit 87.

In accordance with the present invention, the adjustable die 70 hasthree recesses 90, 91, and 92 in its Surface 89 extending from therecovery channel to the exit edge 93 of the die lip. The centerlines ofthe lateral recesses are offset slightly (onef0urth inch in thedescribed embodiment) toward the ends of the die in relation to thecenterline of the strip 31 applied to the web. The recesses do notproject into the exit edge 93 and that edge is therefore spaceduniformly away from the opposed edge 94 of the fixed die lip throughoutthe length of the extruder die. It should be understood that greater orfewer than three recesses could be employed and that the recesses couldbe formed in the fixed die lip or both die lips could be recessedwithout departing from the invention or altering its operation. Theparticular shape of the recesses is important and will be discussed indetail below.

The adjustable die lip 70 is mounted to the die body 63 by a bolt 71 asindicated above. The bolt 71 has a loose fit with respect to theadjustable die lip, thereby permitting transverse movement of the dielip with respect to the axis of the bolt. As best illustrated in FIG.10, the adjustable die lip is connected to a micrometer adjusting screwassembly 100 by a lock pin 101 passing through the die lip 70 and aninside bolt 102. The inside bolt is threaded to the inside of an outsidebolt 103, the outside bolt being threaded to a stationary block 104which is keyed to the die body at 105 and bolted to the die body by abolt 106 as shown in FIG. 8. The connection of the micrometer adjustingscrew assembly to the adjustable die lip coupled with the looseconnection of the die lip to the clamping bolt 71 permits the spacing ofthe die lip with respect to the fixed die lip 65 to be varied within thelimits required in the operation of the extruder.

Referring to FIG. 13, the shape of the recesses 90, 91 and 92 isillustrated. The full length of the recess in the example illustrated isapproximately seven inches. Its length will, of course, depend upon thewidth of the profiled strip to be deposited or extruded onto the web.Over approximately two inches at the center of the recess, the recesshas a surface 95 tapered back from the edge 93 uniformly to a depth of0.009 inch. That portion is designated as A and in transverse sectionthe recess is in the shape of a triangle (FIG. 9).

Each recessed corner designated as B is formed by three triangularsurfaces 110, 111, and 112, all having a common apex on the die edge atthe corner 113 of the recess. At the rearward edge 114 of the recess,the point 115 ou triangle 110 is at the same level as the rearward edgeof the central portion A. Triangles 111 and 112 have a common point 116on rearward edge 114 which is elevated about one-third of the maximumdepth of the recess, that is, the depth of the drop from the die edge 93to the rearward edge 114. The triangle 112 has a third point 117 whichis at the same level as the die edge 93. The central portion A and thetwo corner portions B thus provide a reces above the die edge 93, therecess, in longitudinal section, being flat in the center and at itsends or corners sloping gently up to the level of the die edge 93.

The recess above the exit of the die causes the molten plastic to beextruded in the shape found in cross section in FIG. 14. There, it canbe seen that immediately upon issuing from the die the strip indicatedat 120 is a smoothly curving bead having a substantially greaterthickness through its center than the approximately .020 inch thicknessof the film as indicated at 118 and which extends over the remainingportion of the die exit.

Immediately upon issuing from the die, the surface tension or internalstresses of the film causes the film to bunch or gather at the cornersas indicated at 119 and thus, the thickened strip attains a generallyrectangular configuration. After being applied to the surface of the web39 and passing through the nip, the laminate has the configurationillustrated in FIG. 16 with the thickened portion of the plasticoverlying that area of the `web which will be cut and provided with themajor horizontal score lines by the converter.

As indicated above, the lineal speed of the web is much faster than thelineal rate of extrusion through the die exit (in excess of twenty timesfaster in the embodiment described) and as a consequence the lm will bereduced in thickness by a factor of greater than twenty in the intervalbetween its exit from the die and its deposit on the web. Theillustration in FIGS. 14, 15, and 16 do not reflect this change inthickness and are for illustrative purposes only.

Operation In operation, a roll of paper is received from a papermanufacturer, the roll being approximately seventy-six inches wide. Theroll indicated at 40 in FIG. 4 is mounted on a stand at the upstream endof the extruder line and is passed over the idler rolls 41 and throughthe respective nips 42 and 51 as described above and is wrapped about areel indicated at 54 in FIG. 4.

Pellets of the plastic are fed into the hopper and are driven by a screwtoward the entrance channel of the die. The particular thermoplasticmaterial to be extruded may be any one of a number of differentpolymeric materials which are capable of being extruded as a film, theseincluding polyethylene, polypropylene, nylon, and the vinyls. In theembodiment described, a medium density polyethylene is employed whichhas a specific gravity of 0.930 and a melt index 2.5.

The screw acting on the polyethylene pellets causes them to fuse andform a molten viscous fluid which is forced at approximately 300 p.s.i.and a temperature of 600 F. into the entrance channel 80. The moltenplastic flows first through the distribution manifold, thereby providinga supply of plastic under uniform pressure extending all along thelength of the die. The plastic then is forced through the space betweenthe die body -63 and the fixed die 65 until it flows into the recoverychannel 86 at a fairly uniform rate. The :plastic then flows into therecesses 90, 91, and 92 as Awell as through the remaining space betweenthe fixed and movable dies and then passes through the die exit 87 as amolten film. In passing through the recesses, the triangular shape ofthe recess provides a reduced pressure drop between the recovery channel86 and the die exit 87 and hence a larger volume of plastic issues fromthe exit adjacent the recesses than from the remaining areas. The ratioof volume from the recessed areas to the volume from the remaining areasstays substantially constant over a wide range of extrusion rates.

-` `As stated above, the lateral profiling recesses 90 and 92 have theircenterlines offset a slight distance from the centerline of the profileas it is to be deposited on theA web. This offset permits the surfacetension effects of the t-otal film to drawthe thickened plastic towardthe center of the film so that when it reaches the paperboard web 39,the lateral strips of thicker plastic are at the proper relationship tothe web. The distance of the offset will normally be determinedempiric-ally and will be dependent upon such factors as thecharacteristics of the plastic, its temperature, the distance of the dieexit from the nip, and the like.

In the initial run, fine adjustments along the length of the die openingcan be made through the manipulation of the micrometer adjusting screwassembly 100 in order to attain the desired thicknesses of the filmacross the width of the Ipaperboard web. In the embodiment described,the exit opening of the die is maintained as uniform as possible alongthe length of the die at .020 inch. Since the lineal speed of the web isin excess of twenty times faster than the film is extruded, the .020inch film will be stretched down to approximately 0.00075 inch acrossthe areas surrounding the thicker profiled strips.

After the laminate has been wound on the reel as at 54, it is slit andtrimmed into three rolls of approximately twenty-five inches width andshipped to the converter. At the converter, the web will be printed andscored, the scoring including two major horizontal score lines 29extending along each side of the profiled portions 31 of the film asindicated in FIG. 2. Following scoring, the web is cut into blanks andthe converter normally a.pplies a manufacturers side seam. Thereafter,the blanks are shipped to the dairy where they are formed as cartonswith the bottoms sealed, filled, and sealed at the top.

While the invention has been described with particular reference to themanufacture of a polyethylene coated milk carton, it should be readilyunderstood that the invention is not only applicable to the coating ofother substrates, but also to the manufacture of unsupported lms wherespecific areas of the film are desired t-o be thicker than other areas.For example, in the manufacture of unsupported plastic bags, it would bepossible through the use of the present invention to manufacture a bagin `which the area of the gusset crease is formed of thicker plasticthan the remaining portions of the bag.

In the manufacture of upsupported lms the invention is suitable for -usewith known processes for making unsupported films. These includeextruding from an elongated linear die onto a chill drum -or extrudingfrom a circular die for blown film extrusion, the circular die having adie opening whose circumference is large compared to the width of thedie opening and hence the die may be considered elongated.

We claim:

1. In a die for extruding a thin thermoplastic film having a thick stripover only a portion of its width and having a pair'of longitudinallyextending spaced apart elongated die lips'terminating in uniformlyspaced apart die edges forming the die exit, the improvement comprisingatleast one of said die lips having a recess formed in its surfacefacing the other die lip immediately `adjacent the die edge, said recesshaving a longitudinal dimension which is substantially shorter than thelength of said die lips, whereby to provide said thick strip.

2. A die according to claim 1 in which said recess has gently slopingcorner surfaces.

3. A die according to claim 1 in which-the recess in said die lip istriangular in transverse section.

4. In a die for extruding a thin thermoplastic film having a thick stripover a portion of its width, said die having a pair of long spaced apartlongitudinally extending lips with facing planar surfaces terminating insubstantially uniformly spaced die edges forming the die exit and alongitudinally extending recovery channel spaced upwardly from the dieedges, the improvement comprising, at least one of said dies havinghaving at least one recess formed in .the planar surface immediatelyadjacent the die edge and flaring upwardly into said recovery channel,said recess having a longitudinal dimension substantially shorter thanthe length of said die edges.

5. A die according to claim 4 in which said recess has a at centralsurface and lateral surfaces gently sloping to meet said planar surface.

6. A die according to claim 4 which is especially adapted to extrude aplastic film 0n a paperboard web used in forming milk cartons, said diehaving three said recesses spaced apart for coating three thicker stripsof plastic over areas corresponding to the bottom portions of milkcartons to be formed from said web.

7. A die according to claim 4 wherein said die edges are spaced apartapproximately .020 inch, and wherein said recess fiares into its planarsurface to a depth of .009 inch at said recovery channel.

References Cited UNITED STATES PATENTS 1,661,069 2/ 1928 Hartung.2,091,125 8/ 1937 Stewart. 2,387,718 10/ 1945 Coleman. 2,514,211 7/ 1950Carlson. 2,821,746 2/ 1958 Bicher. 3,074,106 l/196-3 Eberman. 3,142,0907/1964 Hoffman, et al. 3,241,183 3/ 1966 Tyrner.

WILLIAM J. STEPHENSON, Primary Examiner U.S. Cl. X.R.

